1. Field of Invention
This invention relates to nuclear magnetic resonance (called "NMR") image forming apparatus, used, for example, in medical equipment, which determines the distribution of certain nuclei of atoms or the like, in a subject to be examined, from the outside of such subject, by utilizing NMR.
2. Description of the Prior Art
In an NMR image forming apparatus, an image of the inside of a subject to be examined, is obtained by placing the subject within magnetic fields which are generated by supplying electric current to a coil which forms static magnetic fields, and by supplying an electrical current at an RF frequency to an RF coil, which applies RF magnetic fields at a predetermined angle to the subject. The results are processed by a computer. In this case, the intensity of the static magnetic fields should be maintained constant in order to obtain a satisfactory NMR image. For example, when a water cooled type 4 coil ordinary conductive type magnet is used to generate a static magnetic field, the current supplied to the coil which generates the static magnetic field is kept at a predetermined level by the use of a constant current source.
However, even use of a constant current source is not entirely satisfactory since changes in temperature will affect the magnetic field intensity. For example, if there is any change in the temperature of the coils, frame, atmosphere, or cooling water, or any change in the amount of cooling water, the temperatures of the coils and support frame will also be changed. This results in the fluctuation of the intensity of the static magnetic fields due to, for example, the deformation of the coils and the frame caused by heat expansion. This results in a deterioration of the NMR image.